Switching power-supply device having control circuit controlling switching element to turn on-and-off

Abstract

A switching power-supply device, includes a first terminal; a first winding connected to the first terminal; a second winding, which is connected in series to the first winding and is magnetically coupled to the first winding; a first capacitor connected in series to the second winding; a transformer including a primary winding connected in series to the first capacitor and a secondary winding magnetically coupled to the primary winding; a rectifying-and-smoothing circuit connected to the secondary winding; a second terminal, which is connected to an opposite end of the primary winding opposite to a connection end connecting to the first capacitor; a first switching element, which is connected between a connection point of the first winding and the second winding and the second terminal; and a control circuit, which controls first switching element to turn on-and-off.

What is claimed is: 
     
       1. A switching power-supply device, comprising:
 a first terminal configured to connect to one end of a DC power source; 
 a first winding connected to the first terminal; 
 a second winding, which is connected in series to the first winding and is magnetically coupled to the first winding; 
 a first capacitor connected in series to the second winding; 
 a transformer including a primary winding connected in series to the first capacitor and a secondary winding magnetically coupled to the primary winding; 
 a rectifying-and-smoothing circuit connected to the secondary winding for rectifying-and-smoothing a voltage generated in the secondary winding and outputting the voltage to an output terminal; 
 a second terminal, which is configured to connect to another end of the DC power source and is connected to an opposite end of the primary winding opposite to a connection end connected to the first capacitor; 
 a first switching element, which is connected between a connection point of the first winding and the second winding and the second terminal; and 
 a control circuit, which receives the voltage from the output terminal and controls the first switching element to turn on-and-off, 
 wherein while the first switching element is being turned off, the first capacitor is charged by a voltage of the DC power source, and while the first switching element is being turned on, a voltage of the first capacitor and a voltage caused in the second winding are applied to the primary winding. 
 
     
     
       2. The switching power-supply device according to  claim 1 , wherein a number of turns of the first winding is smaller than a number of turns of the second winding. 
     
     
       3. The switching power-supply device according to  claim 1 , further comprising:
 a series circuit, in which a second switching element and a second capacitor are provided in series, wherein the series circuit is connected in parallel to the first switching element, and 
 wherein the control circuit controls one of the first switching element and the second switching element to turn off while the other one of the first switching element and the second switching element is turned on, and 
 wherein the control circuit varies a ratio of a time of turning on the first switching element and a time of turning on the second switching element to control the voltage output from the output terminal.